A Java method that get a number as String and return comma-separated one.

ThousandsCommaSeparator.java

public static String commaDelimited(String number) {
        if (isBlank(number)) {
            return "";
        }
        number = number.replaceAll(",", "");
        
        if (number.length() <= 3) {
            return number;
        }

        final char[] numberCharArray = number.toCharArray();
        final char[] sb = new char[numberCharArray.length + ((numberCharArray.length - 1) / 3)];
        
        int counter = 1;
        int k = sb.length - 1;
        
        for (int i = numberCharArray.length - 1; i >= 0; i--) {
            sb[k] = numberCharArray[i];

            if (counter == 3 && k > 1) {
                sb[--k] = ',';
                counter = 1;
            } else {
                ++counter;
            }
            --k;
        }

        return String.copyValueOf(sb);
    }